1. Field of the Invention
The present invention relates generally to methods of projecting radiation and lens assemblies for photolithography exposure tools and, more particularly, to projection methods and lens assemblies that correct for lens densification due to radiation exposure.
2. Description of Related Art
In integrated circuit device fabrication, photolithographic processes typically project radiation of a predetermined wavelength through a patterned mask called a reticle. The mask has a circuit patterned formed on it. The radiation transmitted through the mask is further transmitted through a reduction lens so that an image of the circuit pattern is projected onto a layer of an energy-sensitive photoresist (or resist) material coated on a semiconductor substrate. The step of transmitting the radiation through the patterned mask thereby transfers an image of the mask pattern into the energy-sensitive material. The image is then developed in the energy-sensitive material and transferred onto the underlying substrate. An integrated circuit device is fabricated using a series of such exposures to pattern different layers formed on a semiconductor substrate.
U.S. Pat. No. 4,474,463 issued to Heimer discloses one relatively early example of a mask illumination system for use in the production of semiconductor devices. The disclosed system includes an optical assembly having an internal focal plane corresponding to the mask. A reticle edge masking assembly (REMA) is located at the internal focal plane and serves to define a pattern of light which is projected onto the mask. The provision of the reticle edge masking assembly in a focal plane separate from the mask plane serves to reduce blurriness caused by near field diffraction, as well as facilitating the use of more complex REMA assemblies.
U.S. Pat. No. 5,982,558 issued to Fxc3xcrter et al. focuses on the REMA objective for a microlithographic projection exposure system. The REMA objective images an object plane onto the reticle plane and has a lens group disposed in the half of the objective close to the reticle. The object plane lies at a finite spacing. In the lens group, the principal ray elevations are greater in magnitude than the elevations of the peripheral rays. A specifically defined scattering surface is arranged in the lens group.
U.S. Pat. No. 5,170,207 issued to Tezuka et al. discloses a projection lens system well-fit for baking integrated circuit patterns onto silicon wafers (or substrates) using a light source having wavelengths ranging from an ultraviolet wavelength zone to a vacuum ultraviolet wavelength zone. This projection lens system is characterized by a plurality of lens elements including a Fresnel lens element having negative dispersion characteristics. The Fresnel lens is located at a particularly defined position in the projection lens system.
U.S. Pat. No. 5,517,279 issued to Hugle et al. focuses on the lenses of a photolithography system. Specifically, Hugle et al. disclose a lens array that can be used as an exposure tool for microlithography. The lens array can be as thin as {fraction (1/40)} of the thickness of a printed page, yet the arrangement of optical lenses can be powerful enough to replace very sophisticated, bulky, and expensive precision optics. The array of lenses can be fabricated with binary optical device and other techniques.
As technology advances, the need to provide more integrated circuit devices on a single chip becomes more pronounced. Consequently, the sizes of the individual devices on a chip are getting smaller. As the sizes of the devices decrease, the wavelength of the radiation used to pattern the energy-sensitive material must also decrease. For devices in the range of 0.35 xcexcm to 0.18 xcexcm, the wavelength of the exposing radiation must be in the range of about 190 nm to about 350 nm, referred to as the deep ultraviolet or deep-UV range.
The use of shorter wavelengths in photolithography poses several challenges in the design of optical components. For example, few optical materials are transparent enough at 193 nm to enable the fabrication of high-quality, all refractive or catadioptric systems as required for lithographic lenses. High-purity synthetic fused silica and crystalline calcium fluoride are currently the only practical choices. Technical difficulties surround the use of calcium fluoride, including a lack of experience grinding and polishing calcium fluoride lenses. Therefore, fused silica is currently the preferred choice for optical lenses in the deep-UV applications. Improvements in production control are needed, however, to supply quality fused silica in production quantities.
U.S. Pat. No. 5,896,222 issued to Rosplock et al. discloses a fused silica lens, a microlithography system including a fused silica lens, and a method of making a fused silica lens. The fused silica lens transmits ultraviolet radiation having a wavelength below 300 nm with controlled optical damage and inhibited red fluorescence during such transmission. The method of manufacturing the lens includes thermally converting a polymethylsiloxane precursor to fused silica particles, consolidating the particles into a body, and forming from the fused silica body an optical lens that transmits ultraviolet radiation, that incurs optical damage up to a certain level when transmitting radiation below a wavelength of 300 nm., that does not incur an absorption transition at any level, that becomes saturated and incurs essentially no significant further damage, and in which the red fluorescence diminishes while further transmitting such radiation.
The use of optical materials in the deep UV regime is not only subject to transparency restrictions, but also to limitations due to the susceptibility of such materials to photo-induced deformation. The physical properties of some materials are altered upon exposure to deep UV radiation. The most serious of these physical alterations is densification. Fused silica exhibits compaction upon extended exposure to 193 nm radiation. Compaction, or densification, is a decrease in the volume of the material with a corresponding increase in the index of refraction in the densified region. Densification of a fused silica lithographic lens causes wavefront distortion. This densification degrades the quality of photolithography in integrated circuit fabrication, resulting in reduced yields and increased costs for semiconductor production.
Because densification of fused silica is a function of radiation exposure, densification is considered the limiting factor on allowable power densities within the lithography field. The densification of the fused silica often used in photolithography processes is a function of the number of laser pulses and the peak intensities of the pulses. Densification increases linearly with the number of pulses and increases quadratically with the peak intensity of the laser pulse.
Excimer laser pulses are extremely short (typically between 5 and 20 ns FWHM). Therefore, high peak intensities are generated by even modest average intensities when excimer laser light sources are used. Excimer laser light sources are very popular because the monochromatic light provides for precise focusing and alignment in photolithographic applications.
U.S. Pat. No. 5,978,070 issued to Sakuma et al. is one example of the incorporation of an excimer laser in a projection exposure apparatus capable of projecting and exposing mask pattern images onto a substrate using an optical projection system. The disclosed apparatus has (1) an optical illumination system capable of illuminating a mask using an excimer laser illuminating light source in a wavelength range of 230 nm or less; and (2) an optical projection system which includes as one optical member a calcium fluoride crystal with a total alkaline earth metal impurity content of 1xc3x971018 atom/cm3 or less and which projects images of the mask pattern onto a substrate. Sakuma et al. also disclose the method by which the calcium fluoride crystal is manufactured.
As a result of the high peak intensities characteristic of excimer lasers, however, considerable optical densification of materials occurs. This optical densification causes wavefront distortion, which in turn causes degraded optical imaging performance resulting in reduced yields and increased costs for semiconductor production. Thus, optical densification limits the useful life of lithographic lenses, and the current solution to this problem is to replace the lenses.
As technology demands the use of shorter wavelength radiation in the photolithography of semiconductor chips, there is a need for optical systems that are optimized for radiation of wavelengths less than 300 nm.
To meet this and other needs, and in view of its purposes, the present invention offers a solution to the problem of densification which otherwise limits the useful lifetime of lithographic lenses. The present invention provides a lens assembly having a first lens element of a material that densifies upon exposure to radiation, and a second lens element of a material that rarefies upon exposure to radiation. The densification or rarefaction of the material results in a corresponding change in the index of refraction of the material. Because this change occurs in opposite directions depending on whether the material densifies or rarefies, a combination of two lenses made with such complementary materials compensates for errors introduced by the change in the refractive index as a result of densification or rarefaction of a lens element when exposed to radiation. Such phenomenon is particularly noticeable when the radiation is laser radiation of a wavelength under 300 nm, especially when an excimer laser radiation of about 193 nm is used.
The present invention also provides a photolithographic exposure apparatus including a lens assembly. The assembly has a first lens element of a material that densifies upon exposure to radiation, and a second lens element of a material that rarefies upon exposure to radiation. The combination of two lenses made with such complementary materials compensates for errors introduced by densification or rarefaction.
The present invention discloses a method of projecting radiation comprising projecting radiation from a first lens element onto a second lens element. An index of refraction of the first lens element and an index of refraction of the second lens element each change upon exposure to radiation. The first lens element index of refraction changes in an opposite direction from a change in the second lens element index of refraction. A method for correcting wavefront distortion due to lens densification in a first lens element is also provided. The method comprises adding a second lens element which rarefies upon exposure to radiation.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention.